OLED/PLEDs have in recent years been actively used in Flat Panel Displays (FPDs) technology. Both organic and polymeric light emitting devices will hereinafter be referred to as OLEDs.
An OLED is made up of at least two electrodes (a cathode and an anode), with an electroluminescent stack sandwiched between the two electrodes. The electroluminescent stack may further comprise of several layers, examples of which are: light emitting layer, hole transport layer and electron transport layer. The working principles of OLEDs are well known in the art and need not be further discussed here.
The electrodes of OLEDs are often made of metal or are formed on rigid or flexible substrates which are both highly reflective. When using such OLEDs in FPDs, this becomes a genuine concern. The metal electrodes of the OLEDs making up the FPD acts collectively like a mirror and disadvantageously reflects ambient light away from the FPD. This causes the FPD to exhibit low visual contrast and poor legibility. Similarly, when electrodes are formed on highly reflective glass-like substrates reflection of ambient light off these glass-like substrates also causes low visual contrast and poor legibility.
Present attempts at reducing this reflection of ambient light in OLEDs are disadvantageously expensive.
Polarizer films have been used to enhance liquid crystal displays to good effect and can be similarly applied for FPDs. However, the adding of the polarizer film constitutes an additional bonding step to the production of the FPD. This aside, polarizer films are subjective to humidity and temperature environments, as such, the operating condition of the FPD is constrained to a limited range of humidity and temperature of the polarizer films. This also results disadvantageously in the inclusion of a material not inherently part of the manufacturing process of the FPD. This eventually results in higher costs.
Aziz et al in US Patent Application Publication No. 2003/0038593A1 teaches an electrically conductive light absorbing layer at the cathode of a bottom emitting OLED. This electrically conductive light absorbing layer serves to absorb reflected ambient light from the cathode and improves the overall contrast and legibility. This light absorbing layer is typically made of a mixture of organic and metal and is placed between the cathode and the organic layer.
There are also other methods using additional light absorbing layers of a variety of different materials. However, they essentially address the reducing of reflected ambient light by incorporating a low reflectivity composite cathode that requires an additional step for device optimization.
It can thus be seen that there exists a need for a simple and effective way for reducing ambient light reflection of OLEDs.